1. Field of Invention
This invention pertains to mechanically formed heat transfer tubes including those employed in various refrigerant condensation and/or boiling applications.
2. Related Art and Other Considerations
Refrigeration apparatus is generally operated both with condenser tubes and boiling (evaporator) tubes. In submerged chiller refrigerating applications, the outside of a boiling tube is submerged in a refrigerant to be boiled, while the inside conveys liquid, usually water, which is chilled as it gives up its heat to the tube and refrigerant. In a condenser tube, on the other hand, refrigerant vapor on the outside of the tube is cooled by a liquid conveyed inside the condenser tube, so that liquid refrigerant condenses on the exterior of the tube and can drip away from the condenser tube.
An outer surface of heat transfer tubes (such as boiling tubes and condenser tubes) typically has fins formed thereon, the fins extending (at least in part) in a direction parallel to a radius of the tube. Heat transfer has also been enhanced by modifying the inner surface of the tube, e.g., by ridges on the tube inner surface, as taught (for example) in U.S. Pat. No. 3,847,212 to Withers, Jr. et al. (incorporated herein by reference). Withers specifically relates an improved heat transfer coefficient to a ridge-dependent severity factor .PHI.=e.sup.2 /pd.sub.i (where e is average height of a ridge, p is the average pitch of the ridges, and d.sub.i is the maximum projected internal diameter of the tube, all measured in inches). Various tubes produced in accordance with the Withers patent have been marketed by Wolverine Tube, Inc., under the trademark TURBO-CHIL.RTM..
FIG. 1 illustrates a portion of a prior art condensing tube 20 manufactured by Wolverine Tube, Inc., and known in the industry as Turbo-CII.TM.. On its tube wall 21, tube 20 of FIG. 1 has fins 22 formed in planes perpendicular to the tube's longitudinal axis 24. Between roots of adjacent fins 22 on the exterior surface 25 of tube 20 is a ravine-shaped channel 26. Channel 26 has a rounded bottom which forms a pocket wherein liquid refrigerant forms in pools 28 during condensation. Eventually the refrigerant which pools in channel 26 is drained away by gravity from an underside of the tube. However, as the depth of the pooled refrigerant 28 increases in channel 26, the distance between tube wall 21 and the refrigerant vapor 29 which is being condensed also increases and constitutes an increased resistance to condensation of more vapor.
Tube 20 also has helical ridges 30 formed on an inner surface 32. Ridges 30 have a predetermined ridge height and pitch and are positioned at a predetermined helix angle. The ridge height e of tube 20 shown in FIG. 1 is 0.015 inch; the inner diameter D.sub.i of tube 20 is 0.612 inch.
Other parameters of interest for tube 20 are the thickness of the tube wall x and the height g of the fins 22 from tube exterior surface 25. For tube 20, x is 0.028 inch and g is 0.040 inch.
The rounded shape of the bottoms of channels 26 results from the configuration of tooling applied to tube exterior surface 25 for forming fins 22. In particular, a series of rotating finning disks (not shown) are employed, in sequence, to apply a force to the tube exterior surface in a manner both to push metal on the exterior outwardly so as to form the fins 22 and to slightly elongate tube 20. Simultaneously, the internal ridges 30 are formed on a mandrel (not shown, which has grooves which are complementary to the ridges). Conventionally, the radially outermost forming portions of the finning disks, as viewed in axial cross section, are rounded (i.e., the finning disks have a profile corresponding to the shape of channels 26). Each of the finning disks in the series has essentially the same rounded profile, with each finning disk in the sequence digging progressively deeper into the tube metal.
It is an object of the present invention to increase heat transfer in boiling and condensing tubes, e.g., by widening and preferably substantially flattening the channels between roots of adjacent fins, thereby reducing the depth of refrigerant pooled in the channels.